This program is aimed at answering two basic questions in developmental biology. (1) How do differentiating cells regulate changing patterns of gene expression during development? (2) How do migrating cells, such as neural crest cells, find their correct paths in the embryo? To address the first question we use the differentiation of murine epidermal cells as a model system, with a focus on the function and regulation of Dlx3, a homeodomain gene transcribed in upper strata of the epidermis. We have shown by gain-of-function experiments that Dlx3 can regulate, possibly directly, the expression of structural genes activated late in the differentiation process. Loss-of-function (gene ablation) studies have revealed that Dlx3 is essential for survival of the mouse embryo beyond 11 days of gestation. A critical function in formation or function of the placenta is hypothesized based on expression data. Studies in yeast and HeLa cells have shown that Dlx3 acts as a positive regulator of transcription. Cooperating factors and target genes are currently being sought by interaction screens and subtractive hybridization methods. The problem of regulating cell migration is being addressed by analyzing the function of a receptor tyrosine kinase, EphA4 (previously Pagliaccio or SEK1). EphA4 is expressed in involuting mesoderm, in a subset of migrating cranial neural crest, and other tissues undergoing migration or rearrangement. It has been shown to play a critical role in the formation of boundaries between segments in the vertebrate hindbrain. We have shown that when this receptor is activated in early Xenopus embryos there is a significant but transient reduction in cell-cell adhesion, probably by modulation of cadherin function. Recent data indicate that this effect on adhesion is mediated at least in part by the src-related tyrosine kinase c-fyn. Further studies on this signaling pathway, and its role in development, are underway.